PRINCIPLES OF FLIGHT

STABILITY AND CONTROL

CHAPTER 3

PRINCIPLES OF FLIGHT

STABILITY AND CONTROL

PLANES AND AXIS

AN AIRCRAFT MOVES AROUND 3 AXIS

LONGITUDINAL

LATERAL

NORMAL

THESE AXIS ARE ALL AT 90° TO EACH OTHER AND PASS THROUGH THE CENTRE OF GRAVITY.

PRINCIPLES OF FLIGHT 3 AXIS

EACH AXIS HAS A PLANE OF OPERATION.WHEN AN AIRCRAFT MOVES IN ONE PLANE IT ROTATES ABOUT AN AXIS.

AN AIRCRAFT CAN MOVE IN 1, 2 OR 3 PLANES AT ANY ONE TIME.

MOVING IN THE PITCHING PLANE ROTATES ABOUT THE LATERAL AXIS.

MOVING IN THE ROLL PLANE ROTATESABOUT THE LONGITUDINAL AXIS.

MOVING IN THE YAWING PLANE ROTATES ABOUT THE NORMAL AXIS.

PLANES

PRINCIPLES OF FLIGHT

PRINCIPLES OF FLIGHT

STABILITY

THIS IS THE ABILITY OF AN AIRCRAFTIN STRAIGHT AND LEVEL FLIGHT TO RECOVER TO IT’S PREVIOUS ATTITUDEWHEN DISTURBED BY TURBULENCE.

LONGITUDINAL STABILITY

THIS OPERATES IN THE PITCHING PLANEAND IS CONTROLLED BY THE TAILPLANE.

THE TAILPLANE IS A LONG WAY FROMTHE CENTRE OF GRAVITY AND THEREFOREHAS A LARGE LEVERAGE AROUND IT.

PRINCIPLES OF FLIGHT

CENTRE OF GRAVITYCENTRE OF GRAVITY

TAILPLANETAILPLANE

LEVERAGELEVERAGE

PRINCIPLES OF FLIGHT LONGITUDINAL STABILITYLONGITUDINAL STABILITY

LIFT PRODUCED ONLIFT PRODUCED ONTAILPLANE PITCHESTAILPLANE PITCHESAIRCRAFT BACK TOAIRCRAFT BACK TOLEVEL FLIGHT.LEVEL FLIGHT.

LIFTLIFT

NOSE UP DISTURBANCENOSE UP DISTURBANCEPRESENTS TAILPLANE ATPRESENTS TAILPLANE ATHIGHER ANGLE OF ATTACKHIGHER ANGLE OF ATTACKTO AIRFLOW.TO AIRFLOW.

NOSE DOWN NOSE DOWN DISTURBANCE PRESENTS DISTURBANCE PRESENTS TAILPLANE AT A TAILPLANE AT A NEGATIVE ANGLE OFNEGATIVE ANGLE OFATTACK TO AIRFLOW.ATTACK TO AIRFLOW.

NEGATIVE LIFT NEGATIVE LIFT PRODUCED ON THEPRODUCED ON THETAILPLANE PITCHES TAILPLANE PITCHES AIRCRAFT BACK TOAIRCRAFT BACK TOLEVEL FLIGHT.LEVEL FLIGHT.

NEGATIVENEGATIVE LIFTLIFT

PRINCIPLES OF FLIGHT

LATERAL STABILITY

THE WINGS ON MOST AIRCRAFT ARE SET ATA SLIGHT UPWARD ANGLE. THIS IS KNOWNAS DIHEDRAL ANGLE.

PRINCIPLES OF FLIGHT DIHEDRAL WINGS

ANDOVER C1

PRINCIPLES OF FLIGHT

IF A WING DROPS IN TURBULENCE, THELIFT NO LONGER ACTS VERTICALLY AND DOESNOT EQUAL THE WEIGHT.

LATERAL STABILITY

THIS CAUSES THE AIRCRAFT TO SLIP SIDEWAYS PUTTING THE LOWER WING AT A GREATER ANGLE OF ATTACK THAN THE HIGHER WING GIVING MORE LIFT TO THE LOWER WING WHICH ROLLS THE AIRCRAFTLEVEL AGAIN.

PRINCIPLES OF FLIGHT

LATERAL STABILITY

ANOTHER EFFECT OF THIS SIDESLIP ISTHE PARTIAL SHIELDING OF THE HIGHER WING FROM THE AIRFLOW – SO REDUCING THE LIFT FURTHER.

SOME AIRCRAFT DO NOT HAVE DIHEDRALWINGS BUT STILL HAVE STABILITY DUE TOTHIS SHIELDING EFFECT.

LATERAL STABILITY

ANOTHER METHOD OF ACHIEVING STABILITYIS BY HAVING THE WING MOUNTED AT THE TOP OF THE FUSELAGE.

THE WEIGHT OF THE AIRCRAFT ACTS BELOW THE LIFT AXIS AND IF THE AIRCRAFT IS DISTURBED CAUSES THE AIRCRAFT TO SWING BACK LIKE A PENDULUM.

THIS IS KNOWN AS THE PENDULUM EFFECT

PRINCIPLES OF FLIGHT

PRINCIPLES OF FLIGHT THE PENDULUM EFFECT

LIFT

LIFT

CENTRE OF GRAVITY

LIFT AXIS

PENDULUM EFFECTMOMMENT

PRINCIPLES OF FLIGHT

LATERAL INSTABILITY

SOMETIMES A DESIGNER WANTS TO REDUCE THE STABILITY OF AN AIRCRAFT.

THIS IS ACHIEVED BY HAVING ANHEDRALANGLED WINGS.

ANHEDRAL WORKS IN THE OPPOSITE SENSETO DIHEDRAL AND IS USED TO COMBAT EXCESSIVE STABILITY IN AN AIRCRAFT DESIGN.

PRINCIPLES OF FLIGHT ANHEDRAL

HARRIER GR9

PRINCIPLES OF FLIGHT DIRECTIONAL STABILITY

IF AN AIRCRAFT IS DISTURBED IN YAW,THE FIN OF THE AIRCRAFT IS PRESENTEDTO THE AIRFLOW AT A DIFFERENT ANGLEAND WILL PRODUCE A WEATHERCOCKING EFFECT.

PRINCIPLES OF FLIGHT STABILITY

THE AMOUNT OF STABILITY REQUIRED BYAN AIRCRAFT WILL DEPEND ON IT’S ROLE.

HIGH STABILITY REQUIRES EXTRA EFFORTTO OVERCOME IT WHEN TRYING TOMANOEUVRE.

LOW STABILITY REQUIRES CONSTANTADJUSTMENT TO MAINTAIN CONTROL.

HIGH STABILITY IS USUALLY FOUND ONLARGE AIRCRAFT WITH A LARGE FIN AREA.

LOW STABILITY IS USUALLY FOUND ON FIGHTER AIRCRAFT WITH A SMALL FIN AREA.